1. Technical Field
The present invention relates to an apparatus for storing gas hydrate pellets, more specifically to an apparatus for storing gas hydrate pellets capable of pulverizing the pellets adhered with one another during the storage and of discharging the pulverized pellets but of adjusting the amount of discharged gas hydrate pellets according to a process speed.
2. Background Art
Natural gas is a clean fossil fuel of which the demand has skyrocketed globally and the resource development has been fiercely competed because it generates significantly smaller quantities of carbon dioxide per fuel mass during the combustion than coal and petroleum.
Natural gas that is produced from gas fields is used as fuel through transportation and storage processes after removing mostly sulfur, carbon dioxide, water and polymer hydrocarbon but methane.
Since the price of natural gas is mostly dependent upon the facility and operation costs of implementing the above processes in addition to the margin and interest, the most economical transportation and storage method is selected, considering various factors such as the size of the gas field and the distance to the consumer. The most typical marine transportation method is the LNG (liquefied natural gas) method, and the compressibility of LNG is about 600 when it is normal condition methane.
Nonetheless, the economic feasibility of the LNG method is restricted due to the cryogenic requirement of LNG, and thus the LNG method is applicable for gas fields larger than a specific scale (i.e., currently at least about 3 trillions of cubic feet).
In order for methane, which is the main component of natural gas, to exist stably as a liquid under atmospheric pressure, the temperature needs to be −162 degrees Celsius or lower. Accordingly, metal materials used in the LNG facility exposed to cryogenic conditions need to include high concentrations of expensive nickel so as to minimize the brittleness. Moreover, due to a great difference in temperature between the inside and the outside during the transportation and storage processes, heat influx causes a large amount of BOG (boil off gas) generation.
In order to achieve economic feasibility of developing relatively small scale gas fields by overcoming these shortcomings and saving production costs of natural gas, GTS (gas to solid) technologies have been widely studied to transport/store natural gas using solid gas hydrate as storage medium. Particularly, in 1990, a Norwegian professor, named Prof. Gudmundsson, presented the self-preservation effect theory of hydrate to motivate many industrialized nations, such as Japan, to develop key technologies required for realizing commercial GTS methods.
Natural gas hydrate (NIGH), which is a crystalline mixture in which natural gas molecules are encapsulated within solid state lattices of hydrogen-bonding water molecules, has an external shape that is similar to ice and maintains its solid state stably if a pressure that is higher than a certain value is applied at a given temperature. In order for methane hydrate to stably exist thermodynamically under atmospheric pressure, the temperatures needs to be −80 degrees Celsius or lower, but the self-preservation effect of delaying the decomposition of hydrate for several weeks is discovered when ice film is formed on the surface of a hydrate particle at temperatures of about −20 degrees Celsius.
The gas compressibility of NGH is about 170 (that is, about 170 cc of normal condition natural gas is stored in 1 cc of hydrate), which is more disadvantageous than LNG, but the temperature condition for transportation and storage of NGH is more advantageous. Accordingly, it has been theoretically verified that the GTS method using NGH is an economically alternative option of the LNG method for small-to-medium scale gas fields.
The GTS technology involves production, transportation/storage and regasification processes. Natural gas produced at gas fields are converted to hydrate under a relatively high-press, low-temperature environment during the production process and then are formed in natural gas hydrate pellets (NGHP) through dehydration, refrigeration, decompression and pellet formation processes. Before the natural gas hydrate pellets are regasified, the natural gas hydrate pellets are transported and stored in pellet forms under the condition of atmospheric pressure and near −20 degrees Celsius.
The conventional pellet storage technology, which is adopted as a process system element in most continuous process lines of various industrial plants, has various types available, depending on the stored material and the use of storage tank. Among these, the silo type, which is a vertical type storage tank having an inlet at the top portion thereof and an outlet at the bottom portion thereof, has a rate of the diameter or width to the height of the storage tank that is 1:1.5 or greater, having a relatively great height rate compared to a bunker. The outlet of the silo-type storage tank may be mostly either a gravity feed type or a rotary feed type. The gravity feed type, which discharges particulate materials by use of falling movements by the self-load of particulate materials, uses a chute having an angle of inclination. The rotary feed type, which is installed in a cylindrical shape of casing and is equipped with a rotor with blades, often uses a rotary feeder that, while the rotor rotates, fills the particulate materials fallen by gravity between the blades and discharges the particulate materials to the bottom.
While the conventional storage tank technology is adopted with the basic concept of the silo tank and the rotary feeder, it still lacks a sufficient technology for readily loading and unloading pellets despite adhesion of the pellets and adhesion between the pellets and the storage tank structure.
The related art is disclosed in Korean Patent Publication No. 2011-0024095 (SILO FOR PELLET TYPE REFUSE DERIVED FUEL; laid open on Mar. 9, 2011).